We often get asked about the inflow capacity of a particular trench drain grate. While this is a valid question and concern, the reality is that the grate inflow is rarely the limiting factor on a trench drain design. The trench drain grates typically have more open area than the channel and outlet pipe can possibly carry. If flow is a concern, the first things that need to be evaluated are the size of the outflow pipe, trench slope, and trench drain internal width.
On short runs of trench or trench grates with small openings the trench grate inflow can be verified with the following formula:
Q = 448.2 * Cd * A * (2*g*h)0.5
where Q = flow rate (gpm), Cd = discharge coefficient (0.6 typically assumed), A = open area of grate, g = gravitational acceleration (32 ft/sec/sec), h = head above grate (in).
Note the primary inflow factors are the open area of the trench grate and the head or amount of water above the grate. In most applications the head will be minimal (1/8" is typically assumed).
This site has a spreadsheet that performs the calculations for you based on the selected grate against the head pressure. If you need help properly evaluating the trench grate inflow feel free to contact us for assistance.